The present invention relates to the field of photolithography used in fabricating semiconductor devices and, more particularly to a method and apparatus for improving etch uniformity in reticle etching by eliminating local etching effects at the edge of the reticle.
In the manufacture of semiconductor wafers, photolithography is used to pattern various layers on a wafer. A layer of resist is deposited on the wafer and exposed using an exposure tool and a template such as a mask or reticle. During the exposure process a form of radiant energy, such as ultraviolet light, is directed through the reticle to selectively expose the resist in a desired pattern. The resist is then developed to remove either the exposed portions, for a positive resist, or the unexposed portions, for a negative resist, thereby forming a resist mask on the wafer. The resist mask can then be used to protect underlying areas of the wafer during subsequent fabrication processes, such as deposition, etching, or ion implantation processes.
An integral component of the photolithographic process is the reticle. The reticle includes the pattern corresponding to features (e.g., transistors or polygates) at a layer of the integrated circuit (IC) design. The reticle may be a transparent glass plate coated with a patterned light blocking material such as, for example, chromium. This type of reticle is typically referred to as a binary mask since light is completely blocked by the light blocking material and fully transmitted through the transparent glass portions.
Another type of reticle is the attenuated phase shift mask (PSM). Attenuated PSMs utilize partially transmissive regions instead of the light blocking regions used in binary masks. The partially transmissive regions typically pass (i.e., do not block) about three to eight percent of the light they receive. Moreover, the partially transmissive regions are designed so that the light that they do pass is shifted by 180 degrees in comparison to the light passing through the transparent (e.g., transmissive) regions.
During the fabrication of reticles, the reticle is often affected by edge effects in the etching chamber. Reference is made to FIGS. 1-3. FIG. 1 shows a plasma etching system 10 including a radio frequency (xe2x80x9cRFxe2x80x9d) source power supply 11, a coil 12, a chamber 13, a dielectric plate 9, a multi-frequency bias power supply 15, and a decoupling capacitor 16. The chamber 13 is connected to a ground potential 17. Reticle 18 is mounted onto electrode 14 which applies a bias voltage or bottom power. Electrode 14 may be an electrostatic-chuck or susceptor for holding the reticle 18 during the etching process. Modulated-bias plasma 19 is generated in chamber 13 from source material 20. Source material may be provided to chamber 13 via one or more feed tubes 52. Reticle has a chrome layer 21 formed thereon and a patterned photoresist layer 22 formed over chrome layer 21. Reticle 18 is reacted with plasma 19 to etch a portion of a surface of chrome layer 21 according to the patterned photoresist 22 to impart the pattern onto the reticle 18. As can be seen from FIGS. 1-3, the reticle 18 is positioned directly over electrode 14. As the plasma bombards the reticle, it etches the reticle on an upper surface as well as at the edges of the reticle 18. FIG. 2 shows a top view of the etched reticle and FIG. 3 shows a cross section of the reticle as shown in FIG. 2. The reticle suffers from edge effects in the etching of the reticle. These edge effects are caused by the existence of the edge of the reticle and the nonuniformity in the reticle formed due to nonuniformity of chemical loading and electrical power at the edge of the reticle. The edge effects may be manifest as a different print quality at the edge of the reticle. Thus, if an integrated circuit pattern extends to the edge of the reticle it will be adversely affected by these edge effects.
There is a need to eliminate edge effects in the reticle to prevent edge anomalies from being transferred onto an integrated circuit or onto a mask used in fabrication of an integrated circuit. This is especially true as feature sizes continue to dramatically decrease, and as the number of features within the IC design continues to increase, it requires reticles which can use a greater portion of the surface for transferring a pattern to an integrated circuit. Accordingly, there is a need and desire for a method and apparatus for eliminating edge effects from the high density etchers in the formation of reticles.
The present invention provides a method and apparatus for improving etch uniformity in reticle etching by eliminating local effects at the edge of the reticle. Specifically, the invention relates to a reticle carrier which surrounds the reticle and is subjected to a plasma etch along with the reticle to reduce edge non-uniformities. The reticle carrier may also be used to move reticles in and out of etch chambers without damaging or contaminating them. To help reduce edge non-uniformities, the reticle carriers are formed of materials similar to that of the reticle and are patterned with a pattern profile similar to that of the reticle.